In recent years, in the medical and graphic arts fields, a decrease in the processing effluent has been increasingly demanded from the viewpoint of environmental protection as well as space saving.
As a result, techniques have been sought which relate to photothermographic materials which can be effectively exposed, employing laser imagers and laser image setters, and can form clear black-and-white images exhibiting high resolution.
Such techniques are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,487,075, both by D. Morgan and B. Shely, or D. H. Klosterboer et al., “Dry Silver Photographic Materials”, (Handbook of Imaging Materials, Marcel Dekker, Inc. page 48, 1991). Also known are silver salt photothermographic dry imaging materials (hereinafter occasionally referred to simply as photothermographic materials) which comprise a support having thereon organic silver salts, photosensitive silver halide and reducing agents.
Since any solution-based processing chemicals are not employed for the aforesaid silver salt photothermographic dry imaging materials, they exhibit advantages in that it is possible to provide a simpler environmentally friendly system to customers.
These silver salt photothermographic dry imaging materials are characterized in that photosensitive silver halide grains, which are incorporated in a photosensitive layer, are utilized as a photo-sensor and images are formed in such a manner that silver halide grains are thermally developed, commonly at 80 to 140° C., utilizing the incorporated reducing agents while using organic silver salts as a supply source of silver ions, and fixing need not be carried out.
However, the aforesaid silver salt photothermographic dry imaging materials tend to result in fogging during storage prior to thermal development, due to incorporation of organic silver salts, photosensitive silver halide grains and reducing agents. Further, after exposure, thermal development is commonly carried out at 80 to 250° C. followed by no fixing. Therefore, since all or some of the silver halide, organic silver salts, and reducing agents remain after thermal development, problems occur in which, during extended storage, image quality such as silver image tone tends to vary due to formation of metallic silver by heat as well as light.
The photothermographic material contains all of the materials required for development in advance, therefore, its shelf-keeping property tends to be lower compared to the conventional photosensitive material for wet development. Further, it is still to be improved in basic properties such as sensitivity and fog. Another problems to be improved is a silver tone in case it is applied to a medical diagnostic use.
There is disclosed a technology to solve the above-described problems by using a silver salt of a dicaroboxylic acid for an organic silver salt (e.g., Patent Document No. 1).
They improve storage stability and image stability after development. Another technology to achieve high speed is also disclosed (e.g., Patent Document No. 2). However, they are not fully sufficient to satisfy all the requirements in the market. Another technology to adjust silver tone yielding a preferable tone is also disclosed. Examples are, Japanese Patent Publication Open to Public Inspection (JP-A) Nos. 50-36110, 59-206831, 5-204087, 11-231460, 2002-169249, and 2002-236334. Here again, the adjusting technologies disclosed are not fully efficient to prevent the color change of the image during preservation.
Patent Document No. 1: JP-A No. 2003-177489
Patent Document No. 2: JP-A No. 2003-140290